This invention relates to a process and apparatus for treating a feed gas. In particular, the invention relates to a thermal swing adsorption (TSA) process using at least three adsorption beds for removing or at least reducing the level of a component in a feed gas to render it suitable for downstream processing and apparatus for use in the process. The invention is especially useful in removing components from a feed gas on a large scale where conventional processes and apparatus are not suitable for use.
Where a feed gas is to be subjected to downstream processing, it may often be desirable or necessary to remove certain components from the feed gas prior to such processing. As an example, high boiling materials for example water and carbon dioxide which may be present in a feed gas, for example air, must be removed where the mixture is to be subsequently treated in a low temperature, for example cryogenic, process. If relatively high boiling materials are not removed, they may liquefy or solidify in subsequent processing and lead to pressure drops, flow difficulties or other disadvantage in the downstream process. Hazardous, for instance explosive, materials are suitably removed prior to further processing of the feed gas so as to reduce the risk of build-up in the subsequent process thereby presenting a hazard. Hydrocarbon gases, for example acetylene, may present such a hazard.
In an air separation process, the gas is typically compressed using a main compressor (MAC) followed by cooling and removal of the thus condensed water in a separator. The gas may be further cooled using for example refrigerated ethylene glycol. The bulk of the water is removed in this step by condensation and separation of the condensate. The gas is then passed to an adsorption process where the components to be removed from the feed gas are removed by adsorption and then to an air separation unit. In treating air, water is conventionally removed first and then carbon dioxide by passing the feed gas through a single adsorbent layer or separate layers of adsorbent selected for preferential adsorption of water and carbon dioxide prior to feeding the treated air to a downstream separation process.
Several methods are known for removing an undesired component from a feed gas by adsorption on to a solid adsorbent including temperature swing adsorption (TSA) and pressure swing adsorption (PSA), thermal pressure swing adsorption (TPSA) and thermally enhanced pressure swing adsorption (TEPSA). Conventionally in such methods, two adsorbent beds are employed in a parallel arrangement with one being operated for adsorption while the other is off-line and being regenerated and then the roles of the beds are periodically reversed in the operating cycle. The adsorption bed is said to be “on-line” during the adsorption step.
In a TSA process, the adsorption step generates heat of adsorption causing a heat pulse to progress downstream through the adsorbent bed. The heat pulse is allowed to proceed out of the downstream end of the adsorbent bed during the feed or on-line period. After adsorption, the flow of feed gas is shut off from the adsorbent bed which is then depressurised. The adsorbent is then exposed to a flow of hot regeneration gas, typically a waste stream or other gas from the downstream process, which strips the adsorbed materials from the adsorbent and so regenerates it for further use. Regeneration conventionally is carried out in a direction counter to that of the adsorption step. The bed is then repressurised in readiness to repeat the adsorption step.
A PSA system typically involves a cycle in which the bed is on-line, and then depressurised, regenerated and then repressurised before being taken back on-line. Depressurisation involves releasing pressurised gas and leads to waste, generally known as “switch loss”. In PSA systems, the pressure of the regeneration gas is lower than that of the feed gas. It is this change in pressure that is used to remove the adsorbed component from the adsorbent. However, cycle times are usually short, for example of the order of 15 to 30 minutes, as compared with those employed in a TSA system, which may be for example of the order of 2 to 20 hours. PSA therefore has certain disadvantages including unacceptable switch loss due to the relatively high frequency of switching between on-line operation and regeneration, especially in operating large capacity plant.
U.S. Pat. No. 5,656,065 describes a PSA process that employs three beds operated in a phased cycle which aims to reduce switch loss and improve continuity of flow of the feed gas to a downstream process. The purpose of the third bed is to allow a process cycle in which a small flow of pressurised feed gas is fed to the bed undergoing repressurisation. Hence, the repressurisation step is relatively long but a reduction in the interruption of the treated gas to a downstream process is advantageously secured.
Thermal pressure swing adsorption (TPSA) is also suitable for removing components from a feed gas by adsorption. In a TPSA system an undesired component is typically adsorbed in a first zone in which an adsorption medium is disposed for example activated alumina or silica gel. A second undesired component is then adsorbed in a second zone. TPSA, utilises a two stage regeneration process in which one adsorbed component is desorbed by TSA and another is desorbed by PSA. A TPSA process is described in U.S. Pat. No. 5,885,650 and U.S. Pat. No. 5,846,295.
In thermally enhanced PSA (TEPSA), desorption occurs by feeding a regeneration gas at a pressure lower than the feed gas and at a temperature greater than it and subsequently replacing the hot regeneration gas by a cold regeneration gas. The heated regeneration gas allows the cycle time to be extended as compared to that of a PSA system so reducing switch losses as heat generated by adsorption within the bed may be replaced in part by the heat from the hot regeneration gas. A TEPSA process is described in U.S. Pat. No. 5,614,000.
TSA, TPSA and TEPSA systems require the input of thermal energy and may require the use of insulated vessels, a regeneration gas preheater and an inlet end precooler and generally the high temperatures impose a more stringent and costly mechanical specification for the system. In operation, there is extra energy cost associated with using the preheater.
By the term “thermal swing adsorption” we mean adsorption processes and apparatus for operating the process in which thermal energy is input to regenerate the adsorbent and includes TPSA and TEPSA processes in addition to TSA unless otherwise stated.
TSA apparatus typically comprises a pair of adsorber vessels, both containing adsorbent. The vessels may be of any conventional type including the vertical, horizontal and radial type.
Conventional purification in a TSA process, especially where larger vessels are employed, may be problematic because the flow characteristics of the gas being processed may place limitations due to the need to avoid undue fluidization of the adsorbent bed and unacceptable pressure drop. In addition complex design of vessel geometry to address these issues, especially to accommodate large flows, may themselves introduce further problems. Accordingly, large vessels present certain problems and there are practical limits for their use.
Radial flow adsorbers have been employed to reduce problems with flow but they are typically more expensive than vertical and simple horizontal vessels. For radial beds, a higher ratio of bed height to bed diameter is required to gain a higher flow. In addition the bed's effective thickness is typically limited by the diameter which may itself be limited by constraints in transporting the adsorption vessel in manufacture and assembly of the plant. Moreover, the bed size is limited by the need to avoid a large pressure drop and lack of uniformity of flow.
In a horizontal bed, a reduction in bed thickness and increase in the ratio of effective bed length to diameter also has practical limits and long horizontal beds are therefore undesirable.
To increase capacity, a “four bed” configuration may be used in a TSA process in which two beds are on line with two beds being regenerated at the same time and the regenerated beds then being placed on-line and the other, exhausted beds being regenerated to provide a high throughput. The four beds are typically operated as two pairs of beds and the phasing of the adsorption/regeneration cycle of the two pairs need not be co-ordinated. In this way four simple vessels having a conventional geometry and design may be used to avoid difficulties of pressure drops and transportation which could be encountered if larger scale equipment were to be employed. This approach however requires significant capital investment and adds to the complexities of design of a large scale separation unit.
U.S. Pat. No. 5,571,309 describes an adsorption process in which a high pressure and a low pressure feed stream are passed to each of a plurality of adsorption beds. The beds are operated in an out of phase cycle. The feed for any given bed is fed sequentially at low and high pressure during a single adsorption cycle and it is necessary to utilise a repressurisation stage just prior to the high and low pressure feed stages. This process seeks to address the problem of providing a product stream at high and low pressure from a single adsorption unit.
The process of U.S. Pat. No. 5,571,309 does not disclose a process for treating a feed gas on a large scale without introducing undue complexity or cost in order to avoid or address the technical difficulties of unacceptably high pressure drop and feed gas flow distribution which are associated with operation of a conventional TSA process on a large scale.